Cellular radio telecommunication systems

ABSTRACT

A private cellular radio telecommunication system is provided that is arranged to support the handout of calls from the private system to an external cellular radio telecommunication system. The private system includes a base station controller that detects when a mobile subscriber unit within the private system with a call in progress, the call in progress not involving the external system, is about to move from the private system into the external system. To facilitate the handout of the call from the private system to the external system the base station controller of the private system sets up a “phantom” call through the external system, the “phantom” call being between the same part as is the call in progress so that as the mobile subscriber unit leaves the private system the phantom call takes over from the call in progress thus accomplishing handouts.

TECHNICAL FIELD

This invention relates to private cellular radio telecommunicationsystems, and especially the interworking of such systems betweenthemselves and with other non-radio telecommunication systems in handingover calls as mobile users move between systems.

Public telecommunication systems are set up to service subscribers overa wide geographical area and are interconnected with other publictelecommunication systems to support calls between subscribers indifferent systems. In public cellular radio telecommunication systems,operators have set up their own networks of radio base stations toprovide coverage for their own mobile subscriber units. Mobilesubscriber units simply register locally with their network when theyswitch on, and their cellular location is then known to the system sothat it can support outgoing calls or incoming calls with subscribers onthe same network or other public telecommunication systems, fixed PSTNor mobile.

Private cellular radio telecommunication systems are set up to serviceusers within a limited area of radio coverage so as to support internalcalls between users. Typically, private systems are set up forin-building coverage.

It would be possible to set up a private cellular system so that itsbase stations are “seen by” an external public cellular system and theassociated cells listed as neighbour cells in the public system. Thisset up will then facilitate the handover of calls between the twosystems as a mobile subscriber unit moves from one to the other.However, the fact that the cells of the private system are likely to bemuch smaller than those of the public system means that the increaseddemand for channels to support these cells and mitigate co-channelinterference problems, is likely to become excessive. However, thedensity of users in a private, in-building network, their relativelysmall size compared to public or macro cells, and their consequentlyhigh number, means that the number of neighbour relationships betweencells to be administered by the public network could be very largeindeed. It is possible, that with current technology, there could bemore than ten times as many in-building cells as macro cells. The numberof neighbour relationships between cells to be administered, maintainedand tested could be up to two orders of magnitude greater than is thecase today. Such a situation places considerable overhead on theOperations and Maintenance Centre (OMC) of any network, and currentlycontributes to the slow acceptance of in-building networks in the worldtoday.

Where private in-building networks have the ability to route calltraffic locally, without reference to the public cellular network, thenan additional problem arises, when a mobile subscriber, engaged in sucha locally routed call wishes to leave the coverage area of the privatenetwork and enter the coverage area of the public network. With currenttechnology, the call is dropped at the point of leaving the privatenetwork, which leaves the mobile subscriber with a negative impressionof the Quality of Service (QoS) they are receiving. The problem arisesbecause with a locally routed call, the public network does not haveresources allocated associated with the call, indeed it has no knowledgeof the call at all, and so is unable to receive the normal handoversignalling that accompanies mobile subscribers as they move about thenetwork.

DISCLOSURE OF THE INVENTION

According to one aspect, the invention consists in a private cellularradio telecommunication system comprising multiple basestations each ofwhich corresponds to a radio cell, at least one of these cells, whichcorresponds to a physical entry point for mobile subscriber units intothe private system, being designated as a “gateway” cell which isregistered with an external macro network, whilst other cells aredesignated as private cells which are not registered with the externalmacro network.

Thus, the demand for channel allocation by the system is reduced, andthe resources provided in the system for new hand-ins can be focussed onthe gateway cell with rapid handover to a private cell to keep thehand-in resource available to the maximum extent possible.

According to a second aspect, the invention consists in a privatecellular radio telecommunication system which is adapted to support thehandout of calls to an external cellular radio telecommunication system,a controller being provided which detects when a mobile subscriber unitwith a call in progress, not involving the external system, is about tomove from the private system into the external system, and sets up a“phantom” call through the external system between the same parties asthe call in progress so that the phantom call takes over from the callin progress as the mobile subscriber unit leaves the private system. Thesoftware agent that manages hand-in and hand-out from the privatenetwork is called the Handover Agent, or HA. This will typically, thoughnot necessarily be executed on a computer local to the private cellularnetwork.

Preferably, the system includes the feature of a gateway cell which isregistered with the external system, and the controller detects whenmobiles enter the gateway cell from within the private network, and actupon this to set up a “phantom” call. The phantom call can then takeover from the call in progress before the mobile exits the gateway celltowards the external system, and conventional handout can then occurbetween the gateway cell and the external system.

DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a schematic drawing of a cellular radio telecommunicationsystem according to the invention as applied to an in-building network,and

FIG. 2 is a schematic drawing showing set up of a phantom call forhand-out to an external public network.

BEST MODE OF CARRYING OUT THE INVENTION

Consider the in-building network shown in FIG. 1. Each basestation BScontains one or more transmitters TX, one of which broadcasts a GSMbeacon, containing the Basestation Control Channel BCCH so that thenetwork is seen by a GSM external public network.

A controller PC is provided which is connected to the basestations via apacket switched local area network LAN and which incorporates a“handover agent” HA which interacts with the public network, so as toallow both hand-in from the public network to the private network, andhand-out from the private network to the public network.

The handover agent HA incorporates a cell planning model in which one ormore basestations close to the physical entrance of the building inwhich the network resides are configured to form one cell called the“gateway cell” G and all other basestations are configured to form asecond cell which is private to the network. The gateway cell G isentered in the public network neighbour cell list, but the private cellis a neighbour to the gateway cell only, and is not entered in theneighbour cell list of the, public network. Thus hand-in occurs into thegateway cell immediately followed by handover to the private cell underthe control of the HA. The radio co-channel resources in the gatewaycell that support hand-in, are therefore the only resources required,and are then kept free for new hand-ins.

An additional benefit of this process is that it prevents unwantedhandover or location update attempts into the network from outside, aspeople inadvertently enter coverage of the network, for instance, bywalking past windows.

Consider a call routed totally on the network between two mobiles MS1and MS2, each of which is located within the radio coverage of arespective basestation BS1 and BS2. Voice traffic between the two isrouted through the basestations BS1 and BS2 over the LAN as directed bya “call agent” CA running on a controller connected to the LAN. This isrepresented as stage 1 in the handout procedure illustrated in FIG. 2.Call setup signalling for this call is omitted for clarity. S1 indicatesspeech or other traffic passing between MS1 and BS1. S2 indicates thetraffic being routed to the opposite end point of the call, between BS1and BS2. S3 indicates the corresponding speech or other traffic passingbetween MS2 and BS2. Note that some protocol conversion can take placeat BS1 and BS2. For instance S1 and S3 may conform to GSM air interfacestandards, but S2 may conform to a VoIP protocol, such as H.323, forexample.

The call agent CA communicates with the basestations, and thebasestations route traffic between each other over the LAN using “avoice over internet protocol” VoIP signalling in accordance with aconventional standard such as H.323, SIP or H.248. The call agentcontrols call set up for mobile-originated calls and mobile-terminatedcalls within the network. It also controls connection of calls via agateway D to a PBX network or IP telephony network.

Whilst an internal call is in progress between the two mobiles MS1 andMS2, the external public network has no knowledge of the call.Therefore, potentially, the call will fail if one of the mobiles leavesthe building/network. In order to address this problem, the handoveragent HA is adapted to detect movement of mobiles from a private cellinto the gateway cell G using an algorithm such as diminishing networksignals or increasing macro network signals. If we take BS1 in FIG. 2 tobe such a gateway cell, then signal S4 indicates to the handover agent(HA) that the mobile subscriber is moving out of coverage of the gatewaycell. The HA can then interrogate the Mobility Management Agent of theprivate network (not shown for clarity) to ascertain if the user ismoving in or out of the private network. In the message sequence of FIG.2, we assume such a signalling transaction indicates that the mobile isexiting the gateway cell, towards the public network. Movement of amobile into the gateway cell G is interpreted as imminent exit from thenetwork, shown as stage 2 in the handout procedure in FIG. 2. Thehandover agent HA therefore establishes a “phantom” call between themobiles MS1 and MS2 over the public network via the gateway cell G,shown at stage 2 in the handout procedure in FIG. 2. The HA uses thesubscription details of MS1 and MS2 (which it has stored during theinitial call setup procedure) in setting up the phantom call. Signals S5to S8 are an abbreviated call setup procedure into the mobile network.Note that some protocol conversion may occur at the PLMN G/W node, totranslate the internal private network signalling into the standardrequired for the PLMN core network. For instance, it may translate H.323signalling into SS7-MAP signalling and vice versa. Alternatively, theinternal private network signalling may be translated into SS7-BSSMAPsignalling and vice versa. Signal S5 is a call setup request signal,phantomed to appear as if it were from MS1. Signal S6 is a page requestsignal towards mobile MS2. The network knows to page MS2 here, since theprivate network will have informed the public network of this at theinitial registration procedure, when MS2 was first switched on in theprivate network. The HA knows that an incoming page request for MS2 isreally part of the phantom call procedure, so it responds with the usualcall progress indications, as if it were MS2. Almost immediately, itwill send the off-hook signal back to the network (S7) as if MS2 hadanswered the call. Then S8 confirms the call connected status of thephantom call. At this stage in the procedure, the HA is handling ontraffic sourcing and sinking. At the successful establishment of thephantom call, the HA informs both BS1 and BS2 (signal S9) that theyshould route their call traffic through the PLMN, rather than to eachother. This phantom call is established by the Base Station ControllerBSC and Mobile Switching Centre MSC in the conventional manner. The HAthen signals the basestations BS1 and BS2 to re-route the speech trafficto the macro network, shown as stage 3 in FIG. 2. The voice traffic isthen re-routed over the public network, and conventional handout takesplace between the gateway cell G and the public network if the mobileleaves the building.

The same handout procedure operates for calls which originate within thenetwork, but terminate outside of the public network; for example, if acall within the network is gatewayed out of the network into a PBXnetwork or the IP telephony network via the gateway D. In that case,replace BS2 with the PBX or IP telephony gateway, and MS2 with the fixedterminal. The same general signalling scheme applies.

1. A private cellular radio telecommunication system which is arrangedto support the handout of calls to an external cellular radiotelecommunication system, the private system including a controllerwhich detects when a mobile subscriber unit with a call in progress, notinvolving the external system, is about to move from the private systeminto the external system, and sets up a “phantom” call through theexternal system between the same parties as the call in progress so thatthe phantom call takes over from the call in progress as the mobilesubscriber unit leaves the private system.
 2. A private cellular radiotelecommunication system according to claim 1, wherein said privatesystem includes a gateway cell registered with the external system, saidcontroller detecting when said mobile subscriber unit enters the gatewaycell and setting up said “phantom” call in response to said detection.3. A private cellular telecommunication system according to anypreceding claim comprising a plurality of basestations connected viasaid controller.
 4. A private cellular telecommunication systemaccording to claim 3, wherein one or more of said basestations areconfigured to form said gateway cell.
 5. A private cellulartelecommunication system according to claim 4, wherein at least one ofthe basestations configured as said gateway cell transmits a controlchannel frequency receivable by mobile subscriber units in said externalsystem.
 6. A private cellular telecommunication system according to anyone of claims 4 to 5, wherein said basestations not configured to formsaid gateway cell are configured to form one or more private cells notregistered with the external system.
 7. A private cellulartelecommunication system according to any preceding claim, wherein saidcontroller comprises a handout agent that controls handout of calls tosaid external system.
 8. A private cellular telecommunication systemaccording to any preceding claim, wherein said controller comprises acall agent that controls call signals within said private system.
 9. Aprivate cellular telecommunication system according to claim 8, whereinsaid call agent performs protocol conversion between a first signallingprotocol used by said private system and a second signalling protocolused by said external system.
 10. A private cellular telecommunicationsystem according to claim 9, wherein said call agent performs saidprotocol conversion when a mobile subscriber is located within saidgateway cell.
 11. A private cellular telecommunication system accordingto claim 9 or 10, wherein said pilot signalling protocol is one ofH.323, SIP or H.248.
 12. A private cellular telecommunication systemaccording to claim 9 or 10, wherein said second signalling protocol isone from SS7-MAP and SS7-BSSMAP.
 13. A private cellulartelecommunication system according to any one of claims 7 to 12, whereinsaid handout agent stores the subscriber details of mobile subscriberunits when a call within said private system is initiated between saidmobile subscriber units.
 14. A private cellular telecommunication systemaccording to claim 13, wherein said handout agent uses said storedsubscriber details to establish said “phantom” call when the detectionof one of said mobile subscriber units entering said gateway celloccurs.
 15. A method of handout from a private cellular radiotelecommunication system to an external cellular radio telecommunicationsystem, said private system comprising a gateway cell corresponding to aphysical entry point into the private system, said gateway cell beingregistered with said external system, and a controller for effectingsaid handout, the method comprising the steps: detecting when a mobilesubscriber with a call in progress enters said gateway cell; and settingup a “phantom” call through the external system between the same partiesas in the call in progress, whereby said “phantom” call takes over thecall in progress as the mobile subscriber leaves the private system. 16.A method according to claim 15, wherein said controller stores thesubscriber details of the mobile subscriber units participating in thecall in progress when said call is set up.
 17. A method according toclaim 16, wherein said controller sets up said “phantom” call throughthe external system by using said stored mobile subscriber details. 18.A method according to any one of claims 15 to 17, wherein saidcontroller performs protocol conversion between a first signallingprotocol used by said private system and a second signalling protocolused by said external system.
 19. A private cellular radiotelecommunication system comprising a plurality of basestations, saidbasestations being configured to form a plurality of cells within saidprivate system, one of said cells corresponding to a physical entrypoint into the private system being designated a gateway cell, whereinsaid gateway cell is registered with an external cellular radiotelecommunication system.